The lactose repressor is a genetic control protein which functions in Escherichia coli by binding to a specific region of the DNA. In response to signal molecules (sugars), a conformational change in the protein occurs, and the repressor no longer binds with the same affinity to the specific DNA site. The lac region of the DNA is thereby freed for the synthesis of mRNA. Elucidation of the molecular events which occur during this process is a necessary prerequisite to understanding the mechanism of induction. The correlation of the structure of the protein with the functions it performs is essential in defining these molecular interactions. Fluorescence techniques provide a means for the determination of structural alterations in response to ligand binding (DNA, signal molecules), for assessing the characteristics of local environments, for determining binding and kinetic parameters for this system, and for measuring interactions between different regions of the protein molecule. By combining observations of intrinsic tryptophan fluorescence in the repressor protein with measurements on fluorophores which can be chemically introduced into the protein, the essential function of genetic control mediated by the repressor can be examined. Insights gained in this fashion may allow predictions concerning other systems and potential progress toward an overview of similar functions in more complex organisms.